There are various kinds of bicycle hydraulic systems. A bicycle hydraulic system (e.g., bicycle hydraulic brake) includes a hydraulic operating device (e.g., brake lever), a hydraulic operated device (e.g., brake caliper), and a hydraulic hose, which connects the hydraulic operating device and the hydraulic operated device. In the hydraulic operating device, pivotal movement of an operation portion (e.g., brake lever arm) moves a piston in a hydraulic chamber. The movement of the piston discharges fluid from the hydraulic chamber. The discharged fluid flows through a hydraulic passage that extends through a base (also referred to as body) of the hydraulic operating device, the hydraulic hose, and a hydraulic passage that extends through a base of the hydraulic operated device to drive a piston located in a hydraulic chamber of the hydraulic operated device.
A wide variety of bicycle hydraulic systems have been developed. This has increased the types of bicycle hydraulic components such as a hydraulic operating device and a hydraulic operated device. Additionally, complicated hydraulic passages are formed in bases of the bicycle hydraulic components. The bases of the bicycle hydraulic components are often bored to form the hydraulic passages. The boring forms an opening that is located in an outer surface of the base. To permanently close the opening and seal the hydraulic passage of the bicycle hydraulic component, a seal member incorporating an O-ring is fitted to the opening from an outer side of the base.